Testing Flex PCBs
Rush PCB ensures the integrity of flexible printed circuit boards ,Testing Flex PCBs through an essential step—testing. We verify the quality of raw materials going in to making the PCBs and the finished products in different ways, such as following the guidelines from IPC-2223, IPC-6013, and IP-FC-234 and their successors. Likewise, for testing flexible printed circuit boards, we follow the IPC-6013D.
IPC published their specification, IPC-6013 in November 1998. The latest edition, IPC-6013D, came in September 2017 with the name “Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards.” The guideline specifies various test methods for flex boards, including thermal, flexing, and impedance testing. The guideline also includes provisions for quality assurance such as sample test coupons, and guidelines for quality conformance and acceptance tests.
IPC-6013D covers much more than just the specifications for processing flex PCBs. As its name suggests, along with flex PCBs, it also covers rigid-flex boards as well. The flex and rigid-flex boards may be single-, double-, or multi-layered, may or may not include PTH, stiffeners, and buried/blind vias. In addition, the flex and or rigid-flex boards may also contain build-up of High Density Interconnect (HDD) layers, with embedded or passive circuitry and distributive capacitance planes, and resistive or capacitive components. The main purpose of the specification is providing requirements for qualification and performance of flex and rigid-flex boards designed to specifications IPC-2221 and IPC-2223.
In general, testing circuit boards extensively depends on the application of the board. There is no single method that fits all boards. Boards meant for aerospace and space applications require a far more demanding testing process than do boards for consumer-electronics.
Bend Test Methods for Flexible Boards
Manufacturing flexible PCBs, whether full or partly flexible, is always a special case. These boards must meet the same quality requirements as for rigid boards, while they must also bend. The tests include quality assurance tests for rigid boards, and a bend test.
The bend test depends on whether the operator will install the board in a flex position, and whether the board will bend only once or flex dynamically. The usage category of the board also determines the minimum bend radius the board must be capable of:
| Installation Type | Minimum Bend Radius |
| One-Time Flexing | 6x – 100x Board Thickness |
| Intermittent Flexing | 6x Board Thickness (1-2 Layers)
12x Board Thickness (≥ 3 Layers) |
| Dynamic Flexing | 100x Board Thickness |
Summary of Test Methods for Rigid-Flex Boards
Regardless of whether the PCB design includes a rigid, flex, or a rigid-flex board, Rush PCB ensures its top-quality performance through adequate testing. Although the testing methods we employ are primarily application dependent, we ensure the testing we perform meets the customer’s desired requirements. To allow our customers determine the tests they can request, we have summarized some common testing methods and the results they must generate regarding board quality:
| Test | Metrics | Results |
| X-Ray | Image | Pass / Fail |
| AOI | Image | Pass / Fail |
| ICT | Impedance | Pass / Fail / Value |
| TDR | Impedance | Value |
| ROSE | Contaminants Present | Value |
According to the above quality assurance tests, we provide definitive results about the quality of PCBs. Based on the above results, customers can easily determine if they need any design or manufacturing changes in their PCBs.
However, the above table only provides common testing methods, and customers and the industry may demand additional tests to cover the more important board attributes for ascertaining the quality of the board.
Bare Board Testing
This is an essential stage before we start assembling the PCB. Bare board testing essentially involves electrically testing the connectivity on the board for opens and shorts. Skipping bare board testing could lead to a risk of having field failures or dead boards. Therefore, with bare board testing, we catch potential problems early on, thereby saving the customer money, time, and effort.
In bare board testing, we test the isolation and continuity of the traces on a bare board before we move on to attach components. The basic requirements are as follows:
Isolation Testing: This involves verifying the required resistance is present between two separate electrical connections.
Continuity Testing: This involves verifying no open points are present between two connected electrical circuits.
By testing the circuit and comparing it to its expected electrical footprint of the design, or netlist, bare board testing ensures the correctness of the circuit part of the PCB. Rush PCB apply these tests with a current limit of 100 mA, and a threshold of 10-50 ohms.
Methods of Bare Board Testing
For a simple board with a few circuits, testing the connections with a digital multimeter would be the best possible solution. However, with the complex multi-layer designs of boards, we must employ more sophisticated methods.
Two common methods suitable for bare board testing involves:
Bed of Nails: This method requires a fixture to hold the PCB in place while testing it with a customized array of spring-loaded pins. Designers pre-determine these pins to connect to specific test points on the board traces, simultaneously testing many points on the board.
Flying Probe Tester: Two or more robotic probes connect to specific nets on the board, testing them. Apart from a fixture to hold the board, no custom fixture is necessary. The operator must program the probes to test and measure specific points on the board.
The Bed of Nails method is suitable for very large-scale production, as preparing the fixture with several pins involves considerable planning, time, and effort. However, the tests take up very little time. Each PCB design requires the creation of a new fixture, and this can be an expensive proposition.
The Flying Probe Tester is slower, but much more adaptable to various PCBs, and more affordable. By changing the program, the operator can easily change over to testing a different board.
Importance of Bare Board Testing
Although it may seem to be unnecessary and time-consuming, Rush PCB carries out bare board testing to guarantee the quality of our flex boards. The overall test set up time is small, the test duration is small, and the investments are minimal. However, the time and effort that we put into bare board testing, we recover many times over by avoiding field testing, repairs, and maintenance costs.
Conclusion
Testing flex PCBs is a part of the quality assurance strategy of Rush PCB we present to our customers. With these tests, we offer a qualitative report of the reliability of our PCBs to our customers. We use a suite of software tools for our PCB design, testing, and analysis to help our customers establish the quality of their board.